This study will investigate whether children with autism differ from non-autistic children on measures of vagal tone. Vagal tone is a measure of respiratory sinus arhythmia. This variability in heart rate is controlled by regions of the brainstem via connections within the tenth cranial nerve (the vagus). Vagal tone is thought to reflect a fundamental part of the human social engagement system. High vagal tone is thought to reflect a readiness to engage with the environment. In addition, changes in vagal tone in response to environmental events indicate the child's attention and or reactivity to these events. This study seeks to better understand the biological bases of autism by examining vagal tone in young children with Autistic Disorder. Past research on cardiac responses in children with autism has not examined vagal tone. Previous research has examined other heart rate measures in autism, but has not contrasted children's reactions to social and nonsocial events. Nor has past research examined relations between cardiac measures and measures of core deficit areas in autism, such as joint attention deficits. In this study, children with autism and a comparison sample will be observed in an experiment that will vary the type of social stimulation in specific ways. The behavioral assay will involve conditions that vary with respect to whether stimuli are social or physical in nature, and whether the events are intrusive or non-intrusive. A central goal for this B-START application will be to refine and calibrate this behavioral assay, as well as to provide an initial test of differences in vagal tone during these conditions. Joint attention behaviors will also be measured. Statistical analyses will test (1) whether children with autism will differ in baseline vagal tone, (2) whether they will differ in the way that vagal tone changes in response to social events, and (3) whether vagal tone is related to joint attention behaviors. This study may provide information on the biological basis of the social deficits in autism and may also help develop methods that can be used to study autism in infancy.